Harry Kane out-jumped Jones in the air. Chris Smalling failed to clear. And Ashley Young went AWOL (absent without official leave), which granted Christian Eriksen enough time to shoot down the optimism around United.

Eriksen's goal in a 2-0 win ended United's run of six games without conceding, but those clean sheets came against Yeovil Town, Burnley, Stoke City, Derby County, Everton and Southampton - a bunch of honest toilers, cup minnows and relegation candidates.

United's last trip to north London provided a more accurate barometer. They prevailed 3-1 against Arsenal, but a seething Arsene Wenger could barely contain his frustration.

Arsenal cultivated 16 shots, but faced a jaw-dropping David de Gea, who treated every shot as a chance to perform his "cat dropped from a ledge" routine.

But Wenger knew. He was aware of United's defensive frailties. When they face opponents of note, they tend to adopt a cautious approach, in fear that they will fold.

Even in defeat, Arsenal managed 16 shots at United's goal. Manchester City scored twice and won. Chelsea only scored once and still triumphed.

Even Watford, Burnley, Bristol City and Leicester City have all netted twice against the Red Devils, offering a more accurate indication of United's defensive regression.

STATS

Do not be fooled by the stats - joint-best defensive record and the highest number of clean sheets in the EPL - for they are the result of Mourinho's safety-first philosophy rather than actual defensive soundness.

Now the transfer window has closed, United's initial giddiness at signing Alexis Sanchez should give away to a more dispassionate analysis.

The Stretford End may mock the Merseysiders for forking out £75 million for Virgil van Dijk, but a similarly bold investment was needed at Old Trafford.

While Manchester City spent £57 million on Aymeric Laporte - despite having a superior back four to begin with - United bought no one apart from Sanchez.

Even a perfunctory examination of their defensive performance against Spurs raises a number of concerns.

PUBLICLY PRAISE

Luke Shaw featured in five of United's last six games and was a redoubtable presence at left back. Mourinho went out of his way to publicly praise Shaw's resurgence.

So, naturally, he dropped him for the first game of real significance against Spurs.

Even for the master of mixed messages, Mourinho's bizarre decision was hard to fathom.

At 32, Young played a ball of wool to Eriksen's restless kitten. He got pushed around for a bit and then brushed aside.

He was almost never used as a fullback by Fergie, and he doesn't look like one now.

And Jones' own goal may look like a rare combination of bad luck and poor timing, but his clumsiness revealed an international defender who doesn't trust his left foot.

In November, Mourinho claimed United couldn't play from the back when Smalling was in the side.

Southgate said much the same when he dropped the defender from the England squad, suggesting that Smalling was unconvincing with the ball at his feet.

He was certainly a liability against Spurs. In fact, he was so jittery and uncertain that Tottenham actively targeted him.

And yet, Smalling endures. Just like Jones.

Eric Bailly will eventually return from injury to settle one position, but the United faithful must be wondering why Mourinho now has a glittering array of attacking superstars and yet continues to rely on a back four that was assembled three managers ago.

With Paul Pogba seemingly incapable of carrying out his defensive duties, Mourinho's decision not to strengthen at the back looks all the more mystifying.

A home fixture against Huddersfield tomorrow shouldn't pose too many problems, but United are off to Sevilla in a few weeks. The European-savvy Spaniards will relish the chance to step back in time and tear into Ferguson's back four.

Neil Humphreys is a British humour columnist and author of three best-selling humorous books about Singapore - Notes From an Even Smaller Island, Scribbles from the Same Island and Final Notes from a Great Island.